This invention relates to self-propelling apparatus for well logging tools, and more particularly to apparatus which functions automatically to first lower a logging tool into a well and thereafter to return the logging tool to the surface.
At the present time, most well logging operations are carried out by means of a wireline. This is considered to be unsatisfactory for a number of reasons. First, with the wireline in use, it is impossible to seal the well in case of a blowout, which is particularly important in off-shore operation. Second, both the wireline and the associated winching are additional apparatus to purchase and use.
Another method of conducting well logging operations is to simply drop the logging tool into the well and then remove it with the drill string. However, this method is so time consuming and costly that it is infeasable except in those instances in which the drill string will be removed irrespective of the logging operation.
Various attempts have heretofore been made at providing a self-propelled well logging tool. In accordance with one prior art system, a well logging tool is permitted to move downwardly at a controlled rate. At the completion of the logging operation, jet propulsion is employed to return the logging tool to the surface.
The foregoing type of self-propelling system for well logging tools is considered to be less than wholly satisfactory from a number of standpoints. First, the system requires a buoyancy section filled with cork or the like in order to partially counteract the weight of the apparatus. This is unsatisfactory because it limits the rate at which the logging tool can travel downwardly through the well. Second, due to its reliance on jet propulsion to return the logging tool to the surface, the prior art system tends to add a considerable amount of gas to well fluid, thereby substantially changing the density of the fluid. This is unsatisfactory because the fluid is typically controlled as to density so as to maintain a predetermined hydrostatic head. The presence of an undue amount of gas in the fluid would tend to lower the hydrostatic head which could in turn lead to a blowout of the well.
It will thus be seen that a need exists for still further improvements in the art of self-propelling apparatus for well logging tools. In particular, a need exists for a self-propelling system for well logging tools which permits the well logging tool to move downwardly in the well as rapidly as may be required for a particular logging operation. There also exists a need for a self-propelling apparatus for well logging tools which does not rely on jet propulsion as the sole means of returning the well logging tool to the surface.
The present invention relates to a self-propelling apparatus for well logging tools which overcomes the foregoing and other problems long since associated with the prior art. In accordance with the broader aspects of the invention, a housing is secured to the housing of a well logging tool, and defines a cylinder. The cylinder is initially filled with a weighted fluid to provide a negative buoyancy, whereby the well logging tool and the self-propelling apparatus attached thereto moved downwardly in the well at a rapid rate under the action of gravity. At the conclusion of the well logging operation, the weighted fluid is expelled from the cylinder to provide a positive buoyancy, whereby the well logging tool and the self-propelling apparatus are rapidly returned to the top of the well. At the top of the well the apparatus is seized by a latching apparatus and is retained for subsequent retrieval of the well logging tool.
In accordance with more specific aspects of the invention, a piston is slidably supported in the cylinder and is normally positioned at one end thereof. A powder charge is mounted in a portion of the housing adjacent to the initial positioning of the piston. The powder charge is adapted for electrical ignition.
At the conclusion of the well logging operation, and in response to the receipt of a signal from the well logging tool, the powder charge in the housing is ignited. This causes the piston to move to the opposite end of the cylinder, thereby expelling the weighted fluid and providing a positive buoyancy. The apparatus is designed such that both the weighted fluid and the expanding gases resulting from the burning of the powder charge flow out of the housing in such a way as to add a jet propulsion effect to the positive buoyancy. The combination of these two effects serve to rapidly return the well logging tool and the self-propelling apparatus to the surface.
In accordance with still other aspects of the invention, the housing is provided with check valves which function to maintain a pressure within the housing which is approximately 500 psi greater than that of the surrounding well fluid. This is advantageous in that since the pressure differential between the interior and the exterior of the housing is maintained at a minimum level, it is not necessary to fabricate the housing from high strength components. This in turn substantially reduces the cost of manufacturing and using the self-propelling apparatus.